Autofocus is one of the technological miracles of the modern age of photography that most of us probably take for granted, but behind that function is some pretty cool tech and science. This great video will give you an informative introduction to how it all works.
Coming to you from ZY Productions, this excellent video explains how phase detection autofocus works. Phase detection can be thought a lot like a miniature automatic rangefinder, but the distinct advantage beyond being automatic is that it's through the lens, meaning there are no parallax errors, and whatever you put the autofocus point on is what the camera will attempt to focus on. Nonetheless, it has certain limitations, and understanding where those come into play, why they happen, and how to work around them can make your life as a photographer a bit easier. Furthermore, the video explains the different types of autofocus points and why certain types are better (and thus, why you should consider not only how many autofocus points a camera has, but how many of each type). This is why you'll notice many top-end bodies touting features like dual cross-type points. Check out the video above for the full breakdown.